📍 Chapter Overview
Haloarenes — Complete Mind Map
Topics Covered: Nature of C-X bond (Resonance, Bond Length) · Preparation (Direct Halogenation, Sandmeyer, Gattermann) · Nucleophilic Substitution (ArSN, Dow's Process) · Effect of NO₂ groups · Electrophilic Substitution (o/p directivity) · Reactions with Metals (Wurtz-Fittig, Fittig) · Environmental issues (DDT, Freons)
🤖 AI Prompt — Chapter Mind Map:
Blue-indigo themed mind map on dark background. Central node: chlorobenzene structure labeled "HALOARENES". Five branches: (1) "C-X Bond Nature" — shows resonance hybriding, 169 pm length, partial double bond character; (2) "Preparation" — shows route from Benzene (Cl2/FeCl3) and Aniline (NaNO2/HCl → Diazonium → Cu2Cl2/HCl); (3) "Nucleophilic Substitution" — box with Dow's process (623K, 300atm) and effect of -NO2 (o/p positions); (4) "Electrophilic Substitution" — 4 boxes: Nitration (HNO3/H2SO4), Sulphonation (oleum), FC Alkylation (CH3Cl/AlCl3), Halogenation (Cl2/FeCl3). Show o/p products; (5) "Metal Reactions" — Fittig (2 phenyl rings join) and Wurtz-Fittig (Alkyl + Phenyl join). High resolution educational poster.
1. Nature of C—X Bond in Haloarenes
Partial Double Bond Character: In haloarenes, the lone pair of electrons on the halogen atom is in conjugation with $\pi$-electrons of the benzene ring. This creates a resonance hybrid where the C—X bond has a partial double bond character.
Bond Length: As a result, the C—Cl bond length in chlorobenzene is **169 pm**, compared to **177 pm** in chloroethane. Shorter bonds are stronger and harder to break.
Hybridization: The C-atom in C—X bond of haloarene is **$sp^2$ hybridized** (more s-character, more electronegative), whereas in haloalkane it is $sp^3$ hybridized. It holds the bonding electron pair more tightly.
Resonance in Chlorobenzene — 5 canonical structures
Draw a resonance diagram of chlorobenzene on a white background. Show 5 canonical structures separated by double-headed arrows. Structure 1: Hexagon with 3 double bonds, Cl at top with 3 lone pairs. Show curved arrow from one lone pair to C1-Cl bond and pi bond to C2. Structure 2: Hexagon with a double bond at C1=Cl, Cl is positive (+), negative charge (-) at Ortho position (C2 or C6). Structure 3: Double bond shifts, negative charge moves to Para position (C4). Structure 4: Charge moves to other Ortho position (C6). Structure 5: Final structure back to neutral chlorobenzene with swapped pi bond positions. Highlight that negative charge density is highest at ORTHO and PARA positions. Label: "Resonance effect in Haloarenes — Partial double bond character". White background, bold structures, textbook quality.
2. Preparation of Haloarenes
2.1 From Hydrocarbons (Direct Halogenation)
Benzene reacts with $Cl_2$ or $Br_2$ in the presence of a Lewis Acid catalyst ($FeCl_3$, $AlCl_3$, or $Fe$ powder in the dark).
2.2 From Diazonium Salts (Most Important NCERT Method)
Sandmeyer's Reaction: Primary aromatic amine is diazotized ($NaNO_2 + HCl, 0–5°C$) to form **Benzene Diazonium Chloride**. This is treated with cuprous halides ($Cu_2Cl_2$ or $Cu_2Br_2$).
- $ArN_2^+Cl^- + Cu_2Cl_2/HCl → ArCl + N_2$
- $ArN_2^+Cl^- + Cu_2Br_2/HBr → ArBr + N_2$
Gattermann Reaction: Uses copper powder ($Cu/HCl$) instead of cuprous salt. (Yield is lower than Sandmeyer).
Balz-Schiemann Reaction: For Fluorobenzene. $ArN_2^+Cl^- + HBF_4 → ArN_2^+BF_4^- \xrightarrow{\Delta} ArF + BF_3 + N_2$.
Iodobenzene: Simply shake diazonium salt with $KI$. No catalyst needed.
3. Chemical Reactions — Nucleophilic Substitution
Why are Haloarenes less reactive towards Nu- substitution? (NCERT points)
- Resonance effect: C—X bond is partial double bond (harder to break).
- Hybridization: $sp^2$ carbon is more electronegative, holding electrons tightly.
- Instability of Phenyl Cation: $C_6H_5^+$ cation is not stabilized by resonance.
- Electronic Repulsion: Electron-rich nucleophile is repelled by the electron-rich $\pi$ cloud of benzene.
3.1 Dow's Process (Industrial Phenol Prep)
Substitution only happens under DRASTIC conditions:
Chlorobenzene + NaOH $\xrightarrow{623K, 300 atm}$ Sodium Phenoxide $\xrightarrow{H^+}$ Phenol
3.2 Presence of Electron Withdrawing Groups (-NO₂)
Reactivity increases dramatically when $-NO_2$ groups are present at Ortho/Para positions.
- **p-nitrochlorobenzene:** Needs 15% $NaOH, 443 K$.
- **o,p-dinitrochlorobenzene:** Needs $Na_2CO_3 (aq), 368 K$.
- **2,4,6-trinitrochlorobenzene (Picryl chloride):** Just warm with **water**.
Mechanism of Nucleophilic Substitution with -NO2 group (ArSN)
Draw the mechanism of hydroxide ion attack on p-nitrochlorobenzene on white background. Step 1: Hydroxide ion (Nu-) attacks the C-Cl carbon. Show the pi bond shifting to the ortho position to form a carbanion intermediate. Step 2 (Stabilization): Draw 3 resonance structures of this carbanion. Show the negative charge landing on C4 (where -NO2 is attached). Draw the resonance where -NO2 oxygen holds the negative charge (-O-N+=O group). Label: "Enhanced stability of carbanion by -NO2 group at para position". Step 3: Fast elimination of Cl-. Product is p-nitrophenol. Add a note: "If -NO2 is at meta, it cannot stabilize the negative charge by resonance, hence reactivity doesn't increase as much". White background, curved arrows, bold labels, textbook style.
4. Electrophilic Substitution Reactions
Halogens are **Deactivating** (due to strong -I effect) but **Ortho-Para Directing** (due to +M effect).
| Reaction | Reagents | Major Product |
|---|
| Nitration | Conc. $HNO_3$ + conc. $H_2SO_4$ | 1-Chloro-4-nitrobenzene (p-isomer) |
| Sulphonation | Conc. $H_2SO_4$ + heat | 4-Chlorobenzenesulphonic acid |
| F.C. Alkylation | $CH_3Cl$ + anhydrous $AlCl_3$ | 1-Chloro-4-methylbenzene |
| F.C. Acylation | $CH_3COCl$ + anhydrous $AlCl_3$ | 4-Chloroacetophenone |
5. Reactions with Metals
Wurtz-Fittig Reaction: Mixture of alkyl halide and aryl halide reacts with sodium in dry ether to form alkylarene.
$$C_6H_5–X + 2Na + R–X \xrightarrow{\text{dry ether}} C_6H_5–R + 2NaX$$
Fittig Reaction: Two molecules of aryl halide react with sodium to form biphenyl.
$$C_6H_5–X + 2Na + X–C_6H_5 \xrightarrow{\text{dry ether}} C_6H_5–C_6H_5 \text{ (Biphenyl)} + 2NaX$$
Wurtz-Fittig and Fittig Reactions — Comparison
Draw two chemical equations on white background. Top equation: Wurtz-Fittig — chlorobenzene + 2Na + methyl chloride. Show a dotted line box around 2Na and the two Cl atoms. Arrow pointing right labeled "dry ether". Product: Toluene (methylbenzene) + 2NaCl. Bottom equation: Fittig — two chlorobenzene molecules + 2Na. Box around 2Na and the two Cl atoms. Arrow pointing right labeled "dry ether". Product: Biphenyl (two connected hexagon rings) + 2NaCl. Label: "Wurtz-Fittig (Alkyl + Aryl)" and "Fittig (Aryl + Aryl)". White background, clean skeletal structures, high contrast.
6. Polyhalogen Compounds
- Dichloromethane: Solvent for paint stripping; affects central nervous system.
- Chloroform ($CHCl_3$): Oxidizes to poisonous **Phosgene ($COCl_2$)** in presence of light/air. Must be kept in dark bottles.
- Iodoform ($CHI_3$): Yellow solid, used as an antiseptic due to liberated $I_2$.
- Carbon Tetrachloride ($CCl₄$): Refrigerant; greenhouse gas; toxic to liver.
- Freons (CFCs): $CF_2Cl_2$ is Freon-12. Stable but depletes ozone layer in stratosphere.
- DDT: p, p'-dichlorodiphenyltrichloroethane. Persistent insecticide, harmful to wildlife. Structure: two chlorobenzene rings connected to a trichloroethane chain.
Ex 1 M: Convert Aniline to Chlorobenzene.
Solution: 1. Diazotisation: Aniline + $NaNO_2$ + $HCl$ ($0-5°C$) → Benzene Diazonium Chloride. 2. Sandmeyer: $N_2^+Cl^-$ + $Cu_2Cl_2/HCl$ → Chlorobenzene.
Ex 2 H: Why is p-nitrochlorobenzene more reactive than chlorobenzene towards NaOH?
Solution: The $-NO_2$ group at para position stabilizes the intermediate carbanion by -M effect. The negative charge in the resonance hybrid can be transferred to the oxygen atoms of the nitro group, lowering the energy of the transition state.
- Using $AgNO_3$ to test for Cl in chlorobenzene — it will **FAILED**! Chlorobenzene doesn't give a precipitate with $AgNO_3$ because the C-Cl bond is too strong.
- Assuming $-NO_2$ at meta position increases reactivity as much as ortho/para — No, the negative charge never reaches the meta position in resonance.
- Writing the electrophile for nitration as $NO_2^-$ — It's $NO_2^+$ (positively charged ion).
- Thinking Fittig is for alkyl halides — Fittig is for Aryl halides ONLY.
